Print engine with adaptive processing

ABSTRACT

A print engine is adapted to print image data from a plurality of pre-processing systems that supply image data at different image resolutions and halftoning states. A data interface receives the image data and associated metadata including an image resolution parameter and a halftone state parameter. A metadata interpreter interprets the metadata and determines image processing operations that are required to prepare the image data for printing using a printer module. A resolution modification processor module processes the image data to modify its resolution if the metadata interpreter determines that the image resolution of the image data does not match the printer resolution. A halftone processor module processes the image data by applying a halftoning operation if the metadata interpreter determines that the image data is not in an appropriate halftoning state.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. ______ (Docket K002076/KES), entitled:“Computational Halftoning Process”, by Kuo et al., which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention pertains to the field of digital printing, and moreparticularly to a print engine adapted to print image data from aplurality of different digital front ends.

BACKGROUND OF THE INVENTION

Electrophotography is a useful process for printing images on a receiver(or “imaging substrate”), such as a piece or sheet of paper or anotherplanar medium (e.g., glass, fabric, metal, or other objects) as will bedescribed below. In this process, an electrostatic latent image isformed on a photoreceptor by uniformly charging the photoreceptor andthen discharging selected areas of the uniform charge to yield anelectrostatic charge pattern corresponding to the desired image (i.e., a“latent image”).

After the latent image is formed, charged toner particles are broughtinto the vicinity of the photoreceptor and are attracted to the latentimage to develop the latent image into a toner image. Note that thetoner image may not be visible to the naked eye depending on thecomposition of the toner particles (e.g., clear toner).

After the latent image is developed into a toner image on thephotoreceptor, a suitable receiver is brought into juxtaposition withthe toner image. A suitable electric field is applied to transfer thetoner particles of the toner image to the receiver to form the desiredprint image on the receiver. The imaging process is typically repeatedmany times with reusable photoreceptors.

The receiver is then removed from its operative association with thephotoreceptor and subjected to heat or pressure to permanently fix(i.e., “fuse”) the print image to the receiver. Plural print images(e.g., separation images of different colors) can be overlaid on thereceiver before fusing to form a multicolor print image on the receiver.

A pre-processing system including a digital front end is typically usedto provide image data to the print engine in a high-speedelectrophotographic printing system. Typically the pre-processing systemis tightly coupled to the print engine such that it is necessary to usean updated pre-processing system to support an updated print engine.There remains a need for a print engine that is capable of interfacingwith pre-processing systems that supply image data having a variety ofimage resolutions and halftoning states.

SUMMARY OF THE INVENTION

The present invention represents a print engine adapted to print imagedata from a plurality of pre-processing systems, wherein thepre-processing systems supply image data at different image resolutionsand halftoning states, including:

a printer module for printing halftoned image data at a printerresolution;

a data interface that receives image data and associated metadata from aparticular pre-processing system, wherein the metadata includes an imageresolution parameter that provides an indication of an image resolutionof the image data provided by the particular pre-processing system and ahalftone state parameter that provides an indication of a halftoningstate of the image data provided by the particular pre-processingsystem;

a metadata interpreter that interprets the metadata and determines imageprocessing operations required to prepare the image data for printingusing the printer module, wherein the metadata interpreter determinesthat a resolution modification operation is required if the metadataindicates that image resolution of the image data does not match theprinter resolution, and wherein the metadata interpreter determines thata halftoning operation is required if the metadata indicates that theimage data is not in an appropriate halftoning state;

a resolution modification processor module that processes the image datato modify the resolution of the image data if the metadata interpreterdetermines that a resolution modification operation is required;

a halftone processor module that processes the image data by applying ahalftoning operation to the image data if the metadata interpreterdetermines that a halftoning operation is required; and

a printer module controller that controls the printer module to producea printed image in accordance with the processed image data.

This invention has the advantage that the print engine is compatiblewith pre-processing systems that provide image data at differentresolutions and in different halftoning states.

It has the additional advantage that the print engine is backwardcompatible with image data provided by previous pre-processing systemversions intended for use with print engines having a lower printerresolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-section of an electrophotographic printersuitable for use with various embodiments;

FIG. 2 is an elevational cross-section of one printing subsystem of theelectrophotographic printer of FIG. 1;

FIG. 3 shows a conventional processing path for producing a printedimage using a pre-processing system couple to a print engine;

FIG. 4 shows an improved processing path including an print engine thatis adapted to produce printed images from image data supplied by avariety of different pre-processing systems;

FIG. 5 shows additional details for the resolution modificationprocessor and the halftone processor of FIG. 4;

FIG. 6 shows a flow chart for a computational halftoning process thatcan be used for the halftoning operation of FIG. 4;

FIG. 7 illustrates a dot shape parameter function useful for thecomputational halftoning process of FIG. 6;

FIG. 8 illustrates a threshold value function useful for thecomputational halftoning process of FIG. 6;

FIG. 9 illustrates an edge softness parameter function useful for thecomputational halftoning process of FIG. 6;

FIG. 10 illustrates example halftoned images formed using thecomputational halftoning process of FIG. 6; and

FIG. 11 is a high-level diagram showing the components of a system forprocessing images in accordance with the present invention.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated, or as are readily apparent to one of skill in the art. Theuse of singular or plural in referring to the “method” or “methods” andthe like is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

As used herein, the terms “parallel” and “perpendicular” have atolerance of ±10°.

As used herein, “sheet” is a discrete piece of media, such as receivermedia for an electrophotographic printer (described below). Sheets havea length and a width. Sheets are folded along fold axes (e.g.,positioned in the center of the sheet in the length dimension, andextending the full width of the sheet). The folded sheet contains two“leaves,” each leaf being that portion of the sheet on one side of thefold axis. The two sides of each leaf are referred to as “pages.” “Face”refers to one side of the sheet, whether before or after folding.

As used herein, “toner particles” are particles of one or morematerial(s) that are transferred by an electrophotographic (EP) printerto a receiver to produce a desired effect or structure (e.g., a printimage, texture, pattern, or coating) on the receiver. Toner particlescan be ground from larger solids, or chemically prepared (e.g.,precipitated from a solution of a pigment and a dispersant using anorganic solvent), as is known in the art. Toner particles can have arange of diameters (e.g., less than 8 μm, on the order of 10-15 μm, upto approximately 30 μm, or larger), where “diameter” preferably refersto the volume-weighted median diameter, as determined by a device suchas a Coulter Multisizer. When practicing this invention, it ispreferable to use larger toner particles (i.e., those having diametersof at least 20 μm) in order to obtain the desirable toner stack heightsthat would enable macroscopic toner relief structures to be formed.

“Toner” refers to a material or mixture that contains toner particles,and that can be used to form an image, pattern, or coating whendeposited on an imaging member including a photoreceptor, aphotoconductor, or an electrostatically-charged or magnetic surface.Toner can be transferred from the imaging member to a receiver. Toner isalso referred to in the art as marking particles, dry ink, or developer,but note that herein “developer” is used differently, as describedbelow. Toner can be a dry mixture of particles or a suspension ofparticles in a liquid toner base.

As mentioned already, toner includes toner particles; it can alsoinclude other types of particles. The particles in toner can be ofvarious types and have various properties. Such properties can includeabsorption of incident electromagnetic radiation (e.g., particlescontaining colorants such as dyes or pigments), absorption of moistureor gasses (e.g., desiccants or getters), suppression of bacterial growth(e.g., biocides, particularly useful in liquid-toner systems), adhesionto the receiver (e.g., binders), electrical conductivity or low magneticreluctance (e.g., metal particles), electrical resistivity, texture,gloss, magnetic remanence, florescence, resistance to etchants, andother properties of additives known in the art.

In single-component or mono-component development systems, “developer”refers to toner alone. In these systems, none, some, or all of theparticles in the toner can themselves be magnetic. However, developer ina mono-component system does not include magnetic carrier particles. Indual-component, two-component, or multi-component development systems,“developer” refers to a mixture including toner particles and magneticcarrier particles, which can be electrically-conductive or-non-conductive. Toner particles can be magnetic or non-magnetic. Thecarrier particles can be larger than the toner particles (e.g., 15-20 μmor 20-300 μm in diameter). A magnetic field is used to move thedeveloper in these systems by exerting a force on the magnetic carrierparticles. The developer is moved into proximity with an imaging memberor transfer member by the magnetic field, and the toner or tonerparticles in the developer are transferred from the developer to themember by an electric field, as will be described further below. Themagnetic carrier particles are not intentionally deposited on the memberby action of the electric field; only the toner is intentionallydeposited. However, magnetic carrier particles, and other particles inthe toner or developer, can be unintentionally transferred to an imagingmember. Developer can include other additives known in the art, such asthose listed above for toner. Toner and carrier particles can besubstantially spherical or non-spherical.

The electrophotographic process can be embodied in devices includingprinters, copiers, scanners, and facsimiles, and analog or digitaldevices, all of which are referred to herein as “printers.” Variousembodiments described herein are useful with electrostatographicprinters such as electrophotographic printers that employ tonerdeveloped on an electrophotographic receiver, and ionographic printersand copiers that do not rely upon an electrophotographic receiver.Electrophotography and ionography are types of electrostatography(printing using electrostatic fields), which is a subset ofelectrography (printing using electric fields). The present inventioncan be practiced using any type of electrographic printing system,including electrophotographic and ionographic printers.

A digital reproduction printing system (“printer”) typically includes adigital front-end processor (DFE), a print engine (also referred to inthe art as a “marking engine”) for applying toner to the receiver, andone or more post-printing finishing system(s) (e.g., a UV coatingsystem, a glosser system, or a laminator system). A printer canreproduce pleasing black-and-white or color images onto a receiver. Aprinter can also produce selected patterns of toner on a receiver, whichpatterns (e.g., surface textures) do not correspond directly to avisible image.

In an embodiment of an electrophotographic modular printing machineuseful with various embodiments (e.g., the NEXPRESS SX 3900 printermanufactured by Eastman Kodak Company of Rochester, N.Y.) color-tonerprint images are made in a plurality of color imaging modules arrangedin tandem, and the print images are successively electrostaticallytransferred to a receiver adhered to a transport web moving through themodules. Colored toners include colorants, (e.g., dyes or pigments)which absorb specific wavelengths of visible light. Commercial machinesof this type typically employ intermediate transfer members in therespective modules for transferring visible images from thephotoreceptor and transferring print images to the receiver. In otherelectrophotographic printers, each visible image is directly transferredto a receiver to form the corresponding print image.

Electrophotographic printers having the capability to also deposit cleartoner using an additional imaging module are also known. The provisionof a clear-toner overcoat to a color print is desirable for providingfeatures such as protecting the print from fingerprints, reducingcertain visual artifacts or providing desired texture or surface finishcharacteristics. Clear toner uses particles that are similar to thetoner particles of the color development stations but without coloredmaterial (e.g., dye or pigment) incorporated into the toner particles.However, a clear-toner overcoat can add cost and reduce color gamut ofthe print; thus, it is desirable to provide for operator/user selectionto determine whether or not a clear-toner overcoat will be applied tothe entire print. A uniform layer of clear toner can be provided. Alayer that varies inversely according to heights of the toner stacks canalso be used to establish level toner stack heights. The respectivecolor toners are deposited one upon the other at respective locations onthe receiver and the height of a respective color toner stack is the sumof the toner heights of each respective color. Uniform stack heightprovides the print with a more even or uniform gloss.

FIGS. 1-2 are elevational cross-sections showing portions of a typicalelectrophotographic printer 100 useful with various embodiments. Printer100 is adapted to produce images, such as single-color images (i.e.,monochrome images), or multicolor images such as CMYK, or pentachrome(five-color) images, on a receiver. Multicolor images are also known as“multi-component” images. One embodiment involves printing using anelectrophotographic print engine having five sets of single-colorimage-producing or image-printing stations or modules arranged intandem, but more or less than five colors can be combined on a singlereceiver. Other electrophotographic writers or printer apparatus canalso be included. Various components of printer 100 are shown asrollers; other configurations are also possible, including belts.

Referring to FIG. 1, printer 100 is an electrophotographic printingapparatus having a number of tandemly-arranged electrophotographicimage-forming printing subsystems 31, 32, 33, 34, 35, also known aselectrophotographic imaging subsystems. Each printing subsystem 31, 32,33, 34, 35 produces a single-color toner image for transfer using arespective transfer subsystem 50 (for clarity, only one is labeled) to areceiver 42 successively moved through the modules. In some embodimentsone or more of the printing subsystem 31, 32, 33, 34, 35 can print acolorless toner image, which can be used to provide a protectiveovercoat or tactile image features. Receiver 42 is transported fromsupply unit 40, which can include active feeding subsystems as known inthe art, into printer 100 using a transport web 81. In variousembodiments, the visible image can be transferred directly from animaging roller to a receiver, or from an imaging roller to one or moretransfer roller(s) or belt(s) in sequence in transfer subsystem 50, andthen to receiver 42. Receiver 42 is, for example, a selected section ofa web or a cut sheet of a planar receiver media such as paper ortransparency film.

In the illustrated embodiments, each receiver 42 can have up to fivesingle-color toner images transferred in registration thereon during asingle pass through the five printing subsystems 31, 32, 33, 34, 35 toform a pentachrome image. As used herein, the term “pentachrome” impliesthat in a print image, combinations of various of the five colors arecombined to form other colors on the receiver at various locations onthe receiver, and that all five colors participate to form processcolors in at least some of the subsets. That is, each of the five colorsof toner can be combined with toner of one or more of the other colorsat a particular location on the receiver to form a color different thanthe colors of the toners combined at that location. In an exemplaryembodiment, printing subsystem 31 forms black (K) print images, printingsubsystem 32 forms yellow (Y) print images, printing subsystem 33 formsmagenta (M) print images, and printing subsystem 34 forms cyan (C) printimages.

Printing subsystem 35 can form a red, blue, green, or other fifth printimage, including an image formed from a clear toner (e.g., one lackingpigment). The four subtractive primary colors, cyan, magenta, yellow,and black, can be combined in various combinations of subsets thereof toform a representative spectrum of colors. The color gamut of a printer(i.e., the range of colors that can be produced by the printer) isdependent upon the materials used and the process used for forming thecolors. The fifth color can therefore be added to improve the colorgamut. In addition to adding to the color gamut, the fifth color canalso be a specialty color toner or spot color, such as for makingproprietary logos or colors that cannot be produced with only CMYKcolors (e.g., metallic, fluorescent, or pearlescent colors), or a cleartoner or tinted toner. Tinted toners absorb less light than theytransmit, but do contain pigments or dyes that move the hue of lightpassing through them towards the hue of the tint. For example, ablue-tinted toner coated on white paper will cause the white paper toappear light blue when viewed under white light, and will cause yellowsprinted under the blue-tinted toner to appear slightly greenish underwhite light.

Receiver 42 a is shown after passing through printing subsystem 31.Print image 38 on receiver 42 a includes unfused toner particles.Subsequent to transfer of the respective print images, overlaid inregistration, one from each of the respective printing subsystems 31,32, 33, 34, 35, receiver 42 a is advanced to a fuser module 60 (i.e., afusing or fixing assembly) to fuse the print image 38 to the receiver 42a. Transport web 81 transports the print-image-carrying receivers to thefuser module 60, which fixes the toner particles to the respectivereceivers, generally by the application of heat and pressure. Thereceivers are serially de-tacked from the transport web 81 to permitthem to feed cleanly into the fuser module 60. The transport web 81 isthen reconditioned for reuse at cleaning station 86 by cleaning andneutralizing the charges on the opposed surfaces of the transport web81. A mechanical cleaning station (not shown) for scraping or vacuumingtoner off transport web 81 can also be used independently or withcleaning station 86. The mechanical cleaning station can be disposedalong the transport web 81 before or after cleaning station 86 in thedirection of rotation of transport web 81.

In the illustrated embodiment, the fuser module 60 includes a heatedfusing roller 62 and an opposing pressure roller 64 that form a fusingnip 66 therebetween. In an embodiment, fuser module 60 also includes arelease fluid application substation 68 that applies release fluid,e.g., silicone oil, to fusing roller 62. Alternatively, wax-containingtoner can be used without applying release fluid to the fusing roller62. Other embodiments of fusers, both contact and non-contact, can beemployed. For example, solvent fixing uses solvents to soften the tonerparticles so they bond with the receiver. Photoflash fusing uses shortbursts of high-frequency electromagnetic radiation (e.g., ultravioletlight) to melt the toner. Radiant fixing uses lower-frequencyelectromagnetic radiation (e.g., infrared light) to more slowly melt thetoner. Microwave fixing uses electromagnetic radiation in the microwaverange to heat the receivers (primarily), thereby causing the tonerparticles to melt by heat conduction, so that the toner is fixed to thereceiver.

The fused receivers (e.g., receiver 42 b carrying fused image 39) aretransported in series from the fuser module 60 along a path either to anoutput tray 69, or back to printing subsystems 31, 32, 33, 34, 35 toform an image on the backside of the receiver (i.e., to form a duplexprint). Receivers 42 b can also be transported to any suitable outputaccessory. For example, an auxiliary fuser or glossing assembly canprovide a clear-toner overcoat. Printer 100 can also include multiplefuser modules 60 to support applications such as overprinting, as knownin the art.

In various embodiments, between the fuser module 60 and the output tray69, receiver 42 b passes through a finisher 70. Finisher 70 performsvarious paper-handling operations, such as folding, stapling,saddle-stitching, collating, and binding.

Printer 100 includes main printer apparatus logic and control unit (LCU)99, which receives input signals from various sensors associated withprinter 100 and sends control signals to various components of printer100. LCU 99 can include a microprocessor incorporating suitable look-uptables and control software executable by the LCU 99. It can alsoinclude a field-programmable gate array (FPGA), programmable logicdevice (PLD), programmable logic controller (PLC) (with a program in,e.g., ladder logic), microcontroller, or other digital control system.LCU 99 can include memory for storing control software and data. In someembodiments, sensors associated with the fuser module 60 provideappropriate signals to the LCU 99. In response to the sensor signals,the LCU 99 issues command and control signals that adjust the heat orpressure within fusing nip 66 and other operating parameters of fusermodule 60. This permits printer 100 to print on receivers of variousthicknesses and surface finishes, such as glossy or matte.

FIG. 2 shows additional details of printing subsystem 31, which isrepresentative of printing subsystems 32, 33, 34, and 35 (FIG. 1).Photoreceptor 206 of imaging member 111 includes a photoconductive layerformed on an electrically conductive substrate. The photoconductivelayer is an insulator in the substantial absence of light so thatelectric charges are retained on its surface. Upon exposure to light,the charge is dissipated. In various embodiments, photoreceptor 206 ispart of, or disposed over, the surface of imaging member 111, which canbe a plate, drum, or belt. Photoreceptors can include a homogeneouslayer of a single material such as vitreous selenium or a compositelayer containing a photoconductor and another material. Photoreceptors206 can also contain multiple layers.

Charging subsystem 210 applies a uniform electrostatic charge tophotoreceptor 206 of imaging member 111. In an exemplary embodiment,charging subsystem 210 includes a wire grid 213 having a selectedvoltage. Additional necessary components provided for control can beassembled about the various process elements of the respective printingsubsystems. Meter 211 measures the uniform electrostatic charge providedby charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating theuniform electrostatic charge on photoreceptor 206 in an image-wisefashion by exposing photoreceptor 206 to electromagnetic radiation toform a latent electrostatic image. The uniformly-charged photoreceptor206 is typically exposed to actinic radiation provided by selectivelyactivating particular light sources in an LED array or a laser deviceoutputting light directed onto photoreceptor 206. In embodiments usinglaser devices, a rotating polygon (not shown) is sometimes used to scanone or more laser beam(s) across the photoreceptor in the fast-scandirection. One pixel site is exposed at a time, and the intensity orduty cycle of the laser beam is varied at each dot site. In embodimentsusing an LED array, the array can include a plurality of LEDs arrangednext to each other in a line, all dot sites in one row of dot sites onthe photoreceptor can be selectively exposed simultaneously, and theintensity or duty cycle of each LED can be varied within a line exposuretime to expose each pixel site in the row during that line exposuretime.

As used herein, an “engine pixel” is the smallest addressable unit onphotoreceptor 206 which the exposure subsystem 220 (e.g., the laser orthe LED) can expose with a selected exposure different from the exposureof another engine pixel. Engine pixels can overlap (e.g., to increaseaddressability in the slow-scan direction). Each engine pixel has acorresponding engine pixel location, and the exposure applied to theengine pixel location is described by an engine pixel level.

The exposure subsystem 220 can be a write-white or write-black system.In a write-white or “charged-area-development” system, the exposuredissipates charge on areas of photoreceptor 206 to which toner shouldnot adhere. Toner particles are charged to be attracted to the chargeremaining on photoreceptor 206. The exposed areas therefore correspondto white areas of a printed page. In a write-black or “discharged-areadevelopment” system, the toner is charged to be attracted to a biasvoltage applied to photoreceptor 206 and repelled from the charge onphotoreceptor 206. Therefore, toner adheres to areas where the charge onphotoreceptor 206 has been dissipated by exposure. The exposed areastherefore correspond to black areas of a printed page.

In the illustrated embodiment, meter 212 is provided to measure thepost-exposure surface potential within a patch area of a latent imageformed from time to time in a non-image area on photoreceptor 206. Othermeters and components can also be included (not shown).

A development station 225 includes toning shell 226, which can berotating or stationary, for applying toner of a selected color to thelatent image on photoreceptor 206 to produce a developed image onphotoreceptor 206 corresponding to the color of toner deposited at thisprinting subsystem 31. Development station 225 is electrically biased bya suitable respective voltage to develop the respective latent image,which voltage can be supplied by a power supply (not shown). Developeris provided to toning shell 226 by a supply system (not shown) such as asupply roller, auger, or belt. Toner is transferred by electrostaticforces from development station 225 to photoreceptor 206. These forcescan include Coulombic forces between charged toner particles and thecharged electrostatic latent image, and Lorentz forces on the chargedtoner particles due to the electric field produced by the bias voltages.

In some embodiments, the development station 225 employs a two-componentdeveloper that includes toner particles and magnetic carrier particles.The exemplary development station 225 includes a magnetic core 227 tocause the magnetic carrier particles near toning shell 226 to form a“magnetic brush,” as known in the electrophotographic art. Magnetic core227 can be stationary or rotating, and can rotate with a speed anddirection the same as or different than the speed and direction oftoning shell 226. Magnetic core 227 can be cylindrical ornon-cylindrical, and can include a single magnet or a plurality ofmagnets or magnetic poles disposed around the circumference of magneticcore 227. Alternatively, magnetic core 227 can include an array ofsolenoids driven to provide a magnetic field of alternating direction.Magnetic core 227 preferably provides a magnetic field of varyingmagnitude and direction around the outer circumference of toning shell226. Development station 225 can also employ a mono-component developercomprising toner, either magnetic or non-magnetic, without separatemagnetic carrier particles.

Transfer subsystem 50 includes transfer backup member 113, andintermediate transfer member 112 for transferring the respective printimage from photoreceptor 206 of imaging member 111 through a firsttransfer nip 201 to surface 216 of intermediate transfer member 112, andthence to a receiver 42 which receives respective toned print images 38from each printing subsystem in superposition to form a composite imagethereon. The print image 38 is, for example, a separation of one color,such as cyan. Receiver 42 is transported by transport web 81. Transferto a receiver is effected by an electrical field provided to transferbackup member 113 by power source 240, which is controlled by LCU 99.Receiver 42 can be any object or surface onto which toner can betransferred from imaging member 111 by application of the electricfield. In this example, receiver 42 is shown prior to entry into asecond transfer nip 202, and receiver 42 a is shown subsequent totransfer of the print image 38 onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from thephotoreceptor 206 to the intermediate transfer member 112, and fromthere to the receiver 42. Registration of the separate toner images isachieved by registering the separate toner images on the receiver 42, asis done with the NexPress 2100. In some embodiments, a single transfermember is used to sequentially transfer toner images from each colorchannel to the receiver 42. In other embodiments, the separate tonerimages can be transferred in register directly from the photoreceptor206 in the respective printing subsystem 31, 32, 33, 34, 25 to thereceiver 42 without using a transfer member. Either transfer process issuitable when practicing this invention. An alternative method oftransferring toner images involves transferring the separate tonerimages, in register, to a transfer member and then transferring theregistered image to a receiver.

LCU 99 sends control signals to the charging subsystem 210, the exposuresubsystem 220, and the respective development station 225 of eachprinting subsystem 31, 32, 33, 34, 35 (FIG. 1), among other components.Each printing subsystem can also have its own respective controller (notshown) coupled to LCU 99.

Various finishing systems can be used to apply features such asprotection, glossing, or binding to the printed images. The finishingsystem scan be implemented as an integral components of the printer 100,or can include one or more separate machines through which the printedimages are fed after they are printed.

FIG. 3 shows a conventional processing path that can be used to producea printed image 450 using a print engine 370. A pre-processing system305 is used to process a page description file 300 to provide image data350 that is in a form that is ready to be printed by the print engine370. In an exemplary configuration, the pre-processing system 305includes a digital front end (DFE) 310 and an image processing module330. The pre-processing system 305 can be a part of printer 100 (FIG.1), or may be a separate system which is remote from the printer 100.The DFE 310 and an image processing module 330 can each include one ormore suitably-programmed computer or logic devices adapted to performoperations appropriate to provide the image data 350.

The DFE 310 receives page description files 300 which define the pagesthat are to be printed. The page description files 300 can be in anyappropriate format (e.g., the well-known Postscript command file formator the PDF file format) that specifies the content of a page in terms oftext, graphics and image objects. The image objects are typicallyprovided by input devices such as scanners, digital cameras or computergenerated graphics systems. The page description file 300 can alsospecify invisible content such as specifications of texture, gloss orprotective coating patterns.

The DFE 310 rasterizes the page description file 300 into image bitmapsfor the print engine to print. The DFE 310 can include variousprocessors, such as a raster image processor (RIP) 315, a colortransform processor 320 and a compression processor 325. It can alsoinclude other processors not shown in FIG. 3, such as an imagepositioning processor or an image storage processor. In someembodiments, the DFE 310 enables a human operator to set up parameterssuch as layout, font, color, media type or post-finishing options.

The RIP 315 rasterizes the objects in the page description file 300 intoan image bitmap including an array of image pixels at an imageresolution that is appropriate for the print engine 370. For text orgraphics objects the RIP 315 will create the image bitmap based on theobject definitions. For image objects, the RIP 315 will resample theimage data to the desired image resolution.

The color transform processor 320 will transform the image data to thecolor space required by the print engine 370, providing colorseparations for each of the color channels (e.g., CMYK). For cases wherethe print engine 370 includes one or more additional colors (e.g., red,blue, green, gray or clear), the color transform processor 320 will alsoprovide color separations for each of the additional color channels. Theobjects defined in the page description file 300 can be in anyappropriate input color space such as sRGB, CIELAB, PCS LAB or CMYK. Insome cases, different objects may be defined using different colorspaces. The color transform processor 320 applies an appropriate colortransform to convert the objects to the device-dependent color space ofthe print engine 370. Methods for creating such color transforms arewell-known in the color management art, and any such method can be usedin accordance with the present invention. Typically, the colortransforms are defined using color management profiles that includemulti-dimensional look-up tables. Input color profiles are used todefine a relationship between the input color space and a profileconnection space (PCS) defined for a color management system (e.g., thewell-known ICC PCS associated with the ICC color management system).Output color profiles define a relationship between the PCS and thedevice-dependent output color space for the printer 100. The colortransform processor 320 transforms the image data using the colormanagement profiles. Typically, the output of the color transformprocessor 320 will be a set of color separations including an array ofpixels for each of the color channels of the print engine 370 stored inmemory buffers.

The processing applied in digital front end 310 can also include otheroperations not shown in FIG. 3. For example, in some configurations, theDFE 310 can apply the halo correction process described incommonly-assigned U.S. Pat. No. 9,147,232 (Kuo) entitled “Reducing haloartifacts in electrophotographic printing systems,” which isincorporated herein by reference.

The image data provided by the digital front end 310 is sent to theimage processing module 330 for further processing. In order to reducethe time needed to transmit the image data, a compressor processor 325is typically used to compress the image data using an appropriatecompression algorithm. In some cases, different compression algorithmscan be applied to different portions of the image data. For example, alossy compression algorithm (e.g., the well-known JPEG algorithm) can beapplied to portions of the image data including image objects, and alossless compression algorithm can be applied to portions of the imagedata including binary text and graphics objects. The compressed imagevalues are then transmitted over a data link to the image processingmodule 330, where they are decompressed using a decompression processor335 which applies corresponding decompression algorithms to thecompressed image data.

A halftone processor 340 is used to apply a halftoning process to theimage data. The halftone processor 340 can apply any appropriatehalftoning process known in the art. Within the context of the presentdisclosure, halftoning processes are applied to a continuous-tone imageto provide an image having a halftone dot structure appropriate forprinting using the printer module 435. The output of the halftoning canbe a binary image or a multi-level image. In an exemplary configuration,the halftone processor 340 applies the halftoning process described incommonly assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled“Multilevel halftone screen and sets thereof,” which is incorporatedherein by reference. For this halftoning process, a three-dimensionalhalftone screen is provided that includes a plurality of planes, eachcorresponding to one or more intensity levels of the input image data.Each plane defines a pattern of output exposure intensity valuescorresponding to the desired halftone pattern. The halftoned pixelvalues are multi-level values at the bit depth appropriate for the printengine 370.

The image enhancement processor 345 can apply a variety of imageprocessing operations. For example, an image enhancement processor 345can be used to apply various image enhancement operations. In someconfigurations, the image enhancement processor 345 can apply analgorithm that modifies the halftone process in edge regions of theimage (see U.S. Pat. No. 7,079,281, entitled “Edge enhancement processorand method with adjustable threshold setting” by Ng et al. and U.S. Pat.No. 7,079,287 entitled “Edge enhancement of gray level images” (both toNg et al.), and both of which are incorporated herein by reference).

The pre-processing system 305 provides the image data 350 to the printengine 370, where it is printed to provide the printed image 450. Thepre-processing system 305 can also provide various signals to the printengine 370 to control the timing at which the image data 350 is printedby the print engine 370. For example, the pre-processing system 305 cansignal the print engine 370 to start printing when a sufficient numberof lines of image data 350 have been processed and buffered to ensurethat the pre-processing system 305 will be capable of keeping up withthe rate at which the print engine 370 can print the image data 350.

A data interface 405 in the print engine 370 receives the data from thepre-processing system 305. The data interface 405 can use any type ofcommunication protocol known in the art, such as standard Ethernetnetwork connections. A printer module controller 430 controls theprinter module 435 in accordance with the received image data 350. In anexemplary configuration, the printer module 435 can be the printer 100of FIG. 1, which includes a plurality of individual electrophotographicprinting subsystems 31, 32, 33, 34, 35 for each of the color channels.For example, the printer module controller 430 can provide appropriatecontrol signals to activate light sources in the exposure subsystem 220(FIG. 2) to exposure the photoreceptor 206 with an exposure pattern. Insome configurations, the printer module controller 430 can apply variousimage enhancement operations to the image data. For example, analgorithm can be applied to compensate for various sources ofnon-uniformity in the printer 100 (e.g., streaks formed in the chargingsubsystem 210, the exposure subsystem 220, the development station 225or the fuser module 60. One such compensation algorithm is described incommonly-assigned U.S. Pat. No. 8,824,907 (Kuo et al.), entitled“Electrophotographic printing with column-dependent tonescaleadjustment,” which is incorporated herein by reference.

In the configuration of FIG. 3, the pre-processing system 305 is tightlycoupled to the print engine 370 in that it must supply image data 350 ina state which is matched to the printer resolution and the halftoningstate required for the printer module 435. As a result, when newversions of the print engine 370 having different printer resolutions orhalftone state requirements are developed, it has been necessary to alsoprovide an updated version of the pre-processing system 305 thatprovides image data 350 in an appropriate state. This has thedisadvantage that customers are required to upgrade both thepre-processing system 305 and the print engine 370 at the same time,both of which can have significant costs. The present inventionaddresses this problem by providing an improved print engine designwhich is compatible with a variety of different pre-processing systems.

Aspects of the present invention will now be described with reference toFIG. 4, which shows an improved print engine 400 that is adapted toproduce printed images 450 from image data 350 provided by a pluralityof different pre-processing systems 305 that are configured to supplyimage data 350 having different image resolutions and halftoning states.In an exemplary configuration, the pre-processing systems 305 aresimilar to that discussed with respect to FIG. 3, and includes a digitalfront end 310 and an image processing module 330. Details of theprocessing provided by the digital front end 310 and an image processingmodule 330 are not included in FIG. 4 for clarity, but will be analogousto the processing operations that were discussed with respect to FIG. 3.In this case, in addition to supplying image data 350, thepre-processing system 305 also supplies appropriate metadata 360 thatprovides an indication of the state of the image data 350. Inparticular, the metadata 360 provides an indication of the imageresolution and the halftoning state of the image data 350.

In an exemplary configuration, the metadata 360 includes an imageresolution parameter that provides an indication of an image resolutionof the image data 350 provided by the pre-processing system 305 and ahalftone state parameter that provides an indication of a halftoningstate of the image data provided by the pre-processing system 305.

The image resolution parameter (R) can take any appropriate form thatconveys information about the image resolution of the image data 350. Insome embodiments, the image resolution parameter can be an integerspecifying the spatial resolution in appropriate units such as dots/inch(dpi) (e.g., R=600 for 600 dpi and R=1200 for 1200 dpi). In otherembodiments, the image resolution parameter can be an index to anenumerated list of allowable spatial resolutions (e.g., R=0 for 600 dpiand R=1 for 1200 dpi).

The halftone state parameter (H) can also take any appropriate form. Insome embodiments, the halftone state parameter can be a Boolean variableindicating whether or not a halftoning process was applied in thepre-processing system 305 such that the image data 350 is in a halftonedstate (e.g., H=FALSE indicates that a halftoning process was not appliedso that the image data 350 is in a continuous tone state, and H=TRUEindicates that a halftoning process was applied sot that the image data350 is in a halftoned state.) In other embodiments, when thepre-processing system 305 applied a halftoning process, the halftonestate parameter can also convey additional information about the type ofhalftoning process that was applied. For example, the halftone stateparameter can be an integer variable, where H=0 indicates that nohalftoning process was applied, and other integer values represent anindex to an enumerated list of available halftoning states (e.g.,different screen frequency/angle/dot shape combinations).

The metadata 360 can also specify other relevant pieces of information.For example, for the case where the image data 350 is in a continuoustone state such that a halftone processor 425 in the print engine 400will be required to apply a halftoning operation, the metadata 360 canalso include one or more halftoning parameters that are used by thehalftone processor 425 to control the halftoning operation. In someembodiments, the halftoning parameters can include a screen angleparameter, a screen frequency parameter, or a screen type parameter. Inother embodiments, the halftoning parameters can include a halftoneconfiguration index that is used to select one of a predefined set ofhalftone algorithm configurations.

The print engine 400 receives the image data 350 and the metadata 360using an appropriate data interface 405 (e.g., an Ethernet interface).The print engine includes a metadata interpreter 410 that analyzes themetadata 360 to provide appropriate control signals 415 that are used tocontrol a resolution modification processor 420 and a halftone processor425, which are used to process the image data 350 to provide processedimage data 428, which is in an appropriate state to be printed by theprinter module 435. Printer module controller 430 then controls theprinter module 435 to print the processed image data 428 to produce theprinted image 450 in an analogous manner to that which was discussedrelative to FIG. 3.

FIG. 5 shows additional details of the resolution modification processor420 and the halftone processor 425 of FIG. 4 according to an exemplaryconfiguration. In this example, the control signals 415 provided by themetadata interpreter 410 (FIG. 4) in response to analyzing the metadata360 (FIG. 4) include a resolution modification flag 416, a resize factor417, a halftoning flag 418 and halftoning parameters 419.

The resolution modification flag 416 provides an indication of whether aresolution modification must be performed. In an exemplary configurationthe resolution modification flag 416 is a Boolean variable that would beset to FALSE if no resolution modification is required (i.e., if theimage resolution of the image data 350 matches the printer resolution ofthe printer module 435), and would be set to TRUE of a resolutionmodification is required.

The halftoning flag 418 provides an indication of whether a halftoningoperation is required. In an exemplary configuration the halftone flag418 is a Boolean variable that would be set to FALSE if no halftoningoperation is required (i.e., if the image data 350 is in a halftoningstate that is appropriate for the printer module 435), and would be setto TRUE if a halftoning operation must be applied to the image data 350before it is ready to be printed.

The resolution modification processor 420 applies modify resolution test421 to determine whether a resolution modification should be performedresponsive to the resolution modification flag 416. If a resolutionmodification is required, a resolution modification operation 422 isperformed. In some configurations, the metadata interpreter 410 (FIG. 4)provides a resize factor 417 that specifies the amount of resizing thatmust be provided to adjust the resolution of the image data 350 to theresolution required by the printer module 435 (FIG. 4). In someconfigurations, the resize factor 417 is a variable specifying the ratiobetween the printer resolution and the image resolution. For example, ifthe image data 350 is at 600 dpi and the printer module 435 prints at1200 dpi, the resize factor 417 would specify that a 2× resolutionmodification is required. In various configurations the resize factor417 could be greater than 1.0 if the printer module 435 has a higherresolution than the image data 350, or it could be less than 1.0 if theprinter module 435 has a lower resolution than the image data 350.

In an exemplary configuration, if the image resolution of the image data350 supplied by the pre-processing system 305 is an integer fraction ofthe printer resolution of the printer module 435 so that the resizefactor 417 is a positive integer, the resolution modification operation422 performs the resolution modification by performing a pixelreplication process. For example, each 600 dpi image pixel in the imagedata 350 would be replaced with a 2×2 array of 1200 dpi image pixels,each having the same pixel value. In other configurations, anappropriate interpolation process can be used by the resolutionmodification operation 422 (e.g., nearest neighbor interpolation,bi-linear interpolation or bi-cubic interpolation). The use of aninterpolation algorithm is particularly useful of the resize factor isnot an integer.

For cases where the resize factor is less than 1.0, the resolutionmodification operation 422 can perform appropriate averaging operationsto avoid aliasing artifacts. For example, if the resize factor 417 is0.5, then 2×2 blocks of image pixels in the image data 350 can beaveraged together to provide the new resolution. In otherconfigurations, the resolution modification operation 422 can apply alow-pass filter operation followed by a resampling operation.

The halftone processor 425 applies halftone image test 426 to determinewhether a halftoning operation should be performed responsive to thehalftoning flag 418. If a halftoning operation is required (e.g., if theimage data 350 is in a continuous-tone state), a halftoning operation427 is performed. In some configurations, the metadata interpreter 410(FIG. 4) provides one or more halftoning parameters 419 that are used tocontrol the halftoning operation. As discussed earlier, the halftoningparameters 419 can include a screen angle parameter, a screen frequencyparameter, or a screen type parameter. In other embodiments, thehalftoning parameters 419 can include a halftone configuration indexthat is used to select one of a predefined set of halftone algorithmconfigurations The halftoning operation applied by the halftoneprocessor 425 can use any appropriate halftoning algorithm known in theart. In some embodiments, any of the halftoning algorithms described incommonly-assigned U.S. Pat. No. 7,218,420 (Tai et al.), entitled “Graylevel halftone processing,” commonly-assigned U.S. Pat. No. 7,626,730(Tai et al.), entitled “Method of making a multilevel halftone screen,”and commonly-assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled,“Multilevel halftone screen and sets thereof,” each of which areincorporated herein by reference, can be used. Such halftoningalgorithms typically involve defining look-up tables defining thehalftone dot shape as a function of position for a tile of pixels.Different look-up tables can be specified to produce different halftonedot patterns. For example, different look-up tables can be specified fordifferent screen angles, screen frequencies and dot shapes. In thiscase, the halftoning parameters 419 can include a halftone configurationindex that selects which look-up table should be used to halftone theimage data 350.

Consider the case where the printer module 435 prints halftoned imagedata at 1200 dpi, but where different pre-processing systems 305 andconfigurations can be used to supply image data 350 at either 600 dpi or1200 dpi, and in either a halftoned state or a continuous tone state. Inthis case, there will be four different combinations of the imageresolution parameters and the halftone state parameters that the printengine must deal with.

-   1. The image resolution parameter indicates that image data 350 is    600 dpi, and the halftone state parameter indicates that the image    data 350 is in a halftoned state. In this case, the print engine 400    would print the image data 350 in a mode that emulates a 600 dpi    printer. The resolution modification processor 420 would be used to    modify the image resolution to provide the 1200 dpi data required by    the printer module 435. In an exemplary embodiment, each 600 dpi    image pixel is replicated to provide a 2×2 array of 1200 dpi image    pixels. Since the image data is already in a halftoned state, the    halftone operation 427 would be bypassed.-   2. The image resolution parameter indicates that the image data 350    is 600 dpi, and the halftone state parameter indicates that the    image data 350 is in a continuous-tone state. In this case, the    resolution modification processor 420 would be used to modify the    image resolution to provide the 1200 dpi data appropriate for the    printer module 435, and the halftone processor 425 would apply a    halftoning operation 427 to the 1200 dpi image data in accordance    with the halftoning parameters 419.-   3. The image resolution parameter indicates that the image data 350    is 1200 dpi, and the halftone state parameter indicates that the    image data 350 is in a halftoned state. In this case, the image data    350 is already in a state that is ready for printing by the printer    module 435, therefore the resolution modification operation 422 and    the halftoning operation 427 would both be bypassed.-   4. The image resolution parameter indicates that image data 350 is    1200 dpi, and the halftone state parameter indicates that the image    data 350 is in a continuous-tone state. In this case, since the    image data is already at 1200 dpi so that the resolution    modification operation 422 would be bypassed, and the halftone    processor 425 would apply a halftoning operation 427 to the 1200 dpi    image data in accordance with the halftoning parameters 419.

In a preferred configuration, the halftone processor 425 uses acomputational halftone process to compute halftoned pixel values using adefined set of calculations. The calculations can be performed in aprocessor (e.g., a field-programmable gate array) located in the printengine 370. For example, the halftone processor 425 can determinehalftoned pixel values E(x,y) for each (x,y) pixel position in the imagedata 350 using the process outlined in FIG. 6. In summary, the processis used to determine halftoned pixels 545 (E(x,y)) using the followingrelationship:

$\begin{matrix}{{E\left( {x,y} \right)} = {\frac{1}{N \times N}{\sum\limits_{i = 1}^{N}{\sum\limits_{j = 1}^{N}{H\left( {u_{i},v_{j}} \right)}}}}} & (1)\end{matrix}$

where the halftoned pixel values E(x,y) define the pattern of exposurevalues to be provided to the photoreceptor 206 (FIG. 2) by the exposuresubsystem 220 (FIG. 2). The halftoned pixel values E(x,y) are determinedby averaging halftoned pixel values computed using a halftone dotfunction 530 (H(u,v)) for an array of high-resolution dot coordinates520 (u_(i),v_(j)) determined by performing a coordinate transformationto an N×N array of high-resolution printer coordinates 510(x_(i),y_(j)). N is a positive integer greater than one. Preferably,N≧3. In an exemplary configuration, N=5. In a preferred embodiment, thehalftone dot function 530 defines a center-growing dot pattern. However,other types of dot patterns can also be used.

One skilled in the art will recognize that Eq. (1) has the effect ofcomputing high-resolution halftoned pixel values at a higher resolutionthan the native printer resolution, and then averaging them down todetermine the final halftoned pixel values E(x,y) at the printerresolution. In this case, high-resolution halftoned pixel values 535(H(u_(i), v_(j))) are determined for an N×N array of sub-pixels for eachinput pixel, such that the high-resolution halftoned exposure values aredetermined at a resolution that is N× greater than the printerresolution.

For each input pixel 500 (C(x,y)) of the image data 350, a definehigh-resolution printer coordinates step 505 is used to define an arrayof high-resolution printer coordinates 510 (x_(i),y_(j)) by sub-samplingthe printer coordinates at a higher resolution than the printerresolution in a neighborhood around the (x,y) pixel position.

In an exemplary configuration, the array of high-resolution printercoordinates 510 can be determined using the following relationship:

x _(i) =x+i/N

y _(j) =y+j/N  (2)

where (x,y) are the pixel coordinates of the input pixel 500 in theprinter coordinate system, and where i and j are array indices whichrange from 0 to N−1. One skilled in the art will recognize that this hasthe effect of defining an N×N array of high-resolution printercoordinates 510 in a neighborhood of the (x,y) pixel coordinate bydefining a set of intermediate positions between the pixel coordinatesof the image data 350. The resulting high-resolution printer coordinates510 are sampled at a higher spatial resolution than the printerresolution associated with the image data 350.

A determine high-resolution dot coordinates step 515 is used todetermine a corresponding array of high-resolution dot coordinates 520(u_(i),v_(j)) by applying an appropriate coordinate transformation tothe p high-resolution printer coordinates 510 (x_(i),y_(j)). Thecoordinate transformation which will be a function of the screen angleand screen frequency of the halftone pattern. In an exemplaryembodiment, the coordinate transformation is performed using thefollowing equation:

$\begin{matrix}{\begin{bmatrix}u_{i} \\v_{j}\end{bmatrix} = {{\begin{bmatrix}{1/\alpha} & 0 \\0 & {1/\alpha}\end{bmatrix}\begin{bmatrix}{\cos (\theta)} & {\sin (\theta)} \\{- {\sin (\theta)}} & {\cos (\theta)}\end{bmatrix}}\begin{bmatrix}x_{i} \\y_{j}\end{bmatrix}}} & (3)\end{matrix}$

where θ=screen angle, and

α=f _(p) /f _(h)  (4)

where f_(h) is the halftone screen frequency (lines/inch), and f_(p) isthe printer resolution (dots/inch). The printer resolution can also bereferred to as pixel frequency of the printer. The first matrix in Eq.(3) scales the printer coordinates in accordance with the ratio α, andthe second matrix in Eq. (3) rotates the printer coordinates inaccordance with the screen angle θ.

A determine high-resolution halftoned pixel values step 525 is used todetermine a set of high-resolution halftoned pixel values 535(H(u_(i),v_(j))) using halftone dot function 530 corresponding to thearray of high-resolution dot coordinates 520 (u_(i),v_(j)). The halftonedot function 530 defines the desired halftone dot value (e.g., theexposure level) as a function of the input image code value C(x,y) andthe relative position within the halftone dot as specified by thehigh-resolution dot coordinates 520. In a preferred embodiment, thehalftone dot function 530 is defined using following relationship:

H(u,v)=T _(E)(h(u,v))  (5)

where h(u,v) is a halftone shape function specifying the halftone dotshape as a function of the dot coordinates (u,v), and T_(E)(h) is anedge shape function. The edge shape function will generally vary as afunction of code value C.

In an exemplary configuration, the halftone shape function h(u,v) can becomputed using the following relationship:

h(u,v)=(u′ ^(p) +v′ ^(p))^(1/p)  (6)

where p is a dot shape parameter (where p=1 produces diamond shaped dotsand p=2 produces circular dots), and where u′ and v′ represent therelative position within the halftone cell, which can be calculated by:

u′=|(u−Int(u))−0.5|

v′=|(v−Int(v))−0.5|  (7)

where Int() is a function that returns the integer portion of a number.Eq. (7) takes advantage of the fact that the dot function will besymmetric around the center of the halftone cell having coordinates(0.5, 0.5).

In some configurations, the value of the dot shape parameter p can be afunction of the code value C so that the dot shape can be variedthroughout the tonescale. FIG. 7 shows an example of a dot shapefunction 600 where the dot shape transitions from circular dots for lowcode values to diamond shaped dots in the mid-tones, and then back tocircular dots for high code values. (The dot shape function 600 in FIG.7 is specified in terms of a normalized code value, which can becomputed by dividing an integer code value by the maximum code value.)

The edge shape function T_(E)(h) maps the halftone dot shape values tocorresponding halftoned pixel values, and can take any appropriate form.At one extreme, the edge shape function can take the form of a hardthreshold. In other cases, the edge shape function can be used toimplement a soft threshold such that the resulting halftone dots willhave a soft edge. In an exemplary configuration, the edge shape functionT(h) can take the following form:

$\begin{matrix}{{T_{E}(h)} = \left\{ \begin{matrix}{1;} & {h < T_{L}} \\{1 - \frac{h - T_{L}}{T_{H} - T_{L}}} & {T_{L} \leq h \leq T_{H}} \\{0;} & {h > T_{H}}\end{matrix} \right.} & (8)\end{matrix}$

where h is the halftone shape value, and T_(L) and T_(H) are low andhigh threshold values, respectfully. In an exemplary configuration, thelow and high threshold values can be computed by:

T _(L) =T(C)−ΔT(C)

T _(H) =T(C)+ΔT(C)  (9)

where T(C) is a threshold value function which indicates where thehalftone function h(u,v) should be thresholded to produce the desiredtone scale, and ΔT(C) is an edge softness function which controls thesoftness of the edges of the halftone dots. Both T(C) and ΔT(C) arefunctions of the image code value (C).

The edge shape function T_(E)(h) given in Eq. (8) includes a lineartransition from the low threshold value T_(L) to the high thresholdvalue T_(H). In alternate configurations, a non-linear function can beused such as a sigmoid function.

The threshold value function T(C) can be used to account variousattributes of the printer 100 (FIG. 1) to produce a desired tonescalerelationship (e.g., density as a function of input code value C). Forexample, the threshold value function T(C) can be specified tocompensate for dot gain characteristics of the print engine 400 (FIG.4). In some embodiments, the threshold value function is determinedusing a calibration process that is performed during a system set-upprocess. FIG. 8 shows an example of a typical threshold value function605 that produces a specified aim tonescale.

FIG. 9 shows examples of edge softness functions ΔT(C) that can be usedto provide halftone dots having different levels of edge softness. Edgesoftness function 610 produces relatively hard edges on the halftonedots, and edge softness function 615 produces relatively soft edges onthe halftone dots.

Returning to a discussion of FIG. 6, once the array of high-resolutionhalftoned pixel values 535 are determined, an average high-resolutionhalftoned pixel values step 540 is used to average the high-resolutionhalftoned pixel values 535 to determine the halftoned pixel value E(x,y)for the halftone pixel 545. The averaging process is reflected by thesummation and normalization shown in Eq. (1).

A more pixels test 550 is then used to determine if all of the pixels inthe image data 350 have been processed. If more pixels remain to beprocessed, the computational halftone process repeats the stepsdescribed above the next input pixel 500.

FIG. 10 shows halftoned images 620, 625 determined using the method ofFIG. 6 and the computations specified in Eqs. (1)-(9). The dot shapefunction 600 of FIG. 7 was used for both of the halftoned images 620,625. Consequently, it can be seen that the circular dots are formed inthe light and dark ends of the tonescale, and that diamond-shaped dotsare formed in the mid-tone portion of the tonescale. The halftoned image620 was formed using the edge softness function 610 of FIG. 8 so thatthe resulting halftone dots have relatively hard edges, and thehalftoned image 625 was formed using the edge softness function 615 ofFIG. 8 so that the resulting halftone dots have relatively hard edges.

It will be obvious to one skilled in the art that various methods can beused to improve the computational efficiency the computations describedabove with reference to FIG. 6. For example, various non-linearfunctions, such as the halftone shape function given in Eq. (6), can beimplemented using look-up tables to avoid the need to performcalculations such as power functions which can be computationallyexpensive.

The use of the computational halftoning process described with referenceto FIG. 6 provides a number of advantages. Most prior art halftoningprocesses are typically limited to processing 8-bit pixel values. Forexample, many halftoning operations utilize a set of look-up tablesdefining the halftone dot shape as a function of position for a tile ofpixels. A different look-up table is often provided for each of 256different input pixel levels. Since the computational halftoning processdescribed herein uses a set of equations rather than a set of predefinedhalftone look-up tables to determine the halftoned pixel values, it caneasily be used to process input pixels having any bit-depth. This beparticularly important where calibration functions are applied to theimage data 350 before applying the halftoning process. If the output ofthe calibration process is limited to an 8-bit value, it has been foundthat the resulting image can be susceptible to quantization artifacts insome cases. As a result, it is preferable that the calibrated image datahave a bit-depth of at least 10 bits.

Another advantage of the computational halftoning process is that it canproduce halftoned images having an arbitrary screen angle and screenfrequency without producing aliasing artifacts. Halftoning operationsthat utilize look-up tables defining the halftone dot shape for a tileof pixels can support only certain screen angle/screen frequencycombinations due to the tiling constraint.

FIG. 11 is a high-level diagram showing the components of a system forprocessing image data according to embodiments of the present invention.The system includes a data processing system 710, a peripheral system720, a user interface system 730, and a data storage system 740. Theperipheral system 720, the user interface system 730 and the datastorage system 740 are communicatively connected to the data processingsystem 710.

The data processing system 710 includes one or more data processingdevices that implement the processes of the various embodiments of thepresent invention, including the example processes described herein. Thephrases “data processing device” or “data processor” are intended toinclude any data processing device, such as a central processing unit(“CPU”), a desktop computer, a laptop computer, a mainframe computer, apersonal digital assistant, a Blackberry™, a digital camera, cellularphone, or any other device for processing data, managing data, orhandling data, whether implemented with electrical, magnetic, optical,biological components, or otherwise. In some embodiments, the dataprocessing system 710 a plurality of data processing devices distributedthroughout various components of the printing system (e.g., thepre-processing system 305 and the print engine 370).

The data storage system 740 includes one or more processor-accessiblememories configured to store information, including the informationneeded to execute the processes of the various embodiments of thepresent invention, including the example processes described herein. Thedata storage system 740 may be a distributed processor-accessible memorysystem including multiple processor-accessible memories communicativelyconnected to the data processing system 710 via a plurality of computersor devices. On the other hand, the data storage system 740 need not be adistributed processor-accessible memory system and, consequently, mayinclude one or more processor-accessible memories located within asingle data processor or device.

The phrase “processor-accessible memory” is intended to include anyprocessor-accessible data storage device, whether volatile ornonvolatile, electronic, magnetic, optical, or otherwise, including butnot limited to, registers, floppy disks, hard disks, Compact Discs,DVDs, flash memories, ROMs, and RAMs.

The phrase “communicatively connected” is intended to include any typeof connection, whether wired or wireless, between devices, dataprocessors, or programs in which data may be communicated. The phrase“communicatively connected” is intended to include a connection betweendevices or programs within a single data processor, a connection betweendevices or programs located in different data processors, and aconnection between devices not located in data processors at all. Inthis regard, although the data storage system 740 is shown separatelyfrom the data processing system 710, one skilled in the art willappreciate that the data storage system 740 may be stored completely orpartially within the data processing system 710. Further in this regard,although the peripheral system 720 and the user interface system 730 areshown separately from the data processing system 710, one skilled in theart will appreciate that one or both of such systems may be storedcompletely or partially within the data processing system 710.

The peripheral system 720 may include one or more devices configured toprovide digital content records to the data processing system 710. Forexample, the peripheral system 720 may include digital still cameras,digital video cameras, cellular phones, or other data processors. Thedata processing system 710, upon receipt of digital content records froma device in the peripheral system 720, may store such digital contentrecords in the data storage system 740.

The user interface system 730 may include a mouse, a keyboard, anothercomputer, or any device or combination of devices from which data isinput to the data processing system 710. In this regard, although theperipheral system 720 is shown separately from the user interface system730, the peripheral system 720 may be included as part of the userinterface system 730.

The user interface system 730 also may include a display device, aprocessor-accessible memory, or any device or combination of devices towhich data is output by the data processing system 710. In this regard,if the user interface system 730 includes a processor-accessible memory,such memory may be part of the data storage system 740 even though theuser interface system 730 and the data storage system 740 are shownseparately in FIG. 11.

A computer program product for performing aspects of the presentinvention can include one or more non-transitory, tangible, computerreadable storage medium, for example; magnetic storage media such asmagnetic disk (such as a floppy disk) or magnetic tape; optical storagemedia such as optical disk, optical tape, or machine readable bar code;solid-state electronic storage devices such as random access memory(RAM), or read-only memory (ROM); or any other physical device or mediaemployed to store a computer program having instructions for controllingone or more computers to practice the method according to the presentinvention.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

PARTS LIST

-   31 printing subsystem-   32 printing subsystem-   33 printing subsystem-   34 printing subsystem-   35 printing subsystem-   38 print image-   39 fused image-   40 supply unit-   42 receiver-   42 a receiver-   42 b receiver-   50 transfer subsystem-   60 fuser module-   62 fusing roller-   64 pressure roller-   66 fusing nip-   68 release fluid application substation-   69 output tray-   70 finisher-   81 transport web-   86 cleaning station-   99 logic and control unit (LCU)-   100 printer-   111 imaging member-   112 intermediate transfer member-   113 transfer backup member-   201 first transfer nip-   202 second transfer nip-   206 photoreceptor-   210 charging subsystem-   211 meter-   212 meter-   213 grid-   216 surface-   220 exposure subsystem-   225 development station-   226 toning shell-   227 magnetic core-   240 power source-   300 page description file-   305 pre-processing system-   310 digital front end (DFE)-   315 raster image processor (RIP)-   320 color transform processor-   325 compression processor-   330 image processing module-   335 decompression processor-   340 halftone processor-   345 image enhancement processor-   350 image data-   360 metadata-   370 print engine-   400 print engine-   405 data interface-   410 metadata interpreter-   415 control signals-   416 resolution modification flag-   417 resize factor-   418 halftoning flag-   419 halftoning parameters-   420 resolution modification processor-   421 modify resolution test-   422 resolution modification operation-   425 halftone processor-   426 halftone image test-   427 halftoning operation-   428 processed image data-   430 printer module controller-   435 printer module-   450 printed image-   500 input pixel-   505 define high-resolution printer coordinates step-   510 high-resolution printer coordinates-   515 determine high-resolution dot coordinates step-   520 high-resolution dot coordinates-   525 determine high-resolution halftoned pixel values step-   530 halftone dot function-   535 high-resolution halftoned pixel values-   540 average high-resolution halftoned pixel values step-   545 halftoned pixel-   550 more pixels test-   600 dot shape function-   605 threshold value function-   610 edge softness function-   615 edge softness function-   620 halftoned image-   625 halftoned image-   710 data processing system-   720 peripheral system-   730 user interface system-   740 data storage system

1. A print engine adapted to print image data from a plurality ofpre-processing systems, wherein the pre-processing systems supply imagedata at different image resolutions and halftoning states, comprising: aprinter module for printing halftoned image data at a printerresolution; a data interface that receives image data and associatedmetadata from a particular pre-processing system, wherein the metadataincludes an image resolution parameter that provides an indication of animage resolution of the image data provided by the particularpre-processing system and a halftone state parameter that provides anindication of a halftoning state of the image data provided by theparticular pre-processing system; a metadata interpreter that interpretsthe metadata and determines image processing operations required toprepare the image data for printing using the printer module, whereinthe metadata interpreter determines that a resolution modificationoperation is required if the metadata indicates that image resolution ofthe image data does not match the printer resolution, and wherein themetadata interpreter determines that a halftoning operation is requiredif the metadata indicates that the image data is not in an appropriatehalftoning state; a resolution modification processor module thatprocesses the image data to modify the resolution of the image data ifthe metadata interpreter determines that a resolution modificationoperation is required; a halftone processor module that processes theimage data by applying a halftoning operation to the image data if themetadata interpreter determines that a halftoning operation is required;and a printer module controller that controls the printer module toproduce a printed image in accordance with the processed image data. 2.The print engine of claim 1, wherein the metadata further includes oneor more halftoning parameters that are used by the halftone processormodule to control the halftoning operation.
 3. The print engine of claim2, wherein the halftoning parameters include a screen angle parameter, ascreen frequency parameter, or a screen type parameter.
 4. The printengine of claim 2, wherein the halftoning parameters include a halftoneconfiguration index that is used to select one of a predefined set ofhalftone algorithm configurations.
 5. The print engine of claim 1,wherein the halftone state parameter is a variable indicating whether ornot the image data has been halftoned.
 6. The print engine of claim 1,wherein the print engine is adapted to print image data having an imageresolution selected from a set of predefined image resolutions, andwherein the image resolution parameter is an image resolution indexindicating which of the predefined image resolutions the image data has.7. The print engine of claim 1, wherein the image resolution supplied bythe pre-processing systems is an integer fraction of the printerresolution, and wherein the resolution modification processor modifiesthe resolution of the image data using a pixel replication process. 8.The print engine of claim 1, wherein the resolution modificationprocessor modifies the resolution of the image data using aninterpolation process.
 9. The print engine of claim 1, wherein themetadata interpreter determines that a halftoning operation is notrequired if the metadata indicates that the received image data is in ahalftoned state.
 10. The print engine of claim 1, wherein the halftoningoperation applies a multi-level halftoning process.
 11. The print engineof claim 1, wherein if the metadata indicates that the image resolutionof the received image data is lower than the printer resolution and thatthe received image data is in a halftoned state, then the print engineis configured to emulate a print engine having a resolution equal to theimage resolution.
 12. The print engine of claim 1, wherein the printermodule is an electrophotographic printing system.